It is well known that internal combustion engines have revolutionized transportation following their invention during the last decades of the 19th century. While others, including Benz and Dailmer, invented and developed engines using electric ignition of fuel such as gasoline, Rudolf C. K. Diesel invented and built an engine named for him which employs compression for autoignition of the fuel in order to utilize low-cost organic fuels. Development of improved diesel engines for use in automobiles has proceeded hand-in-hand with improvements in diesel fuel compositions which are today typically derived from petroleum. Modern high performance diesel engines demand ever more advanced specification of fuel compositions, but cost remains an important consideration.
As alternatives to conventional hydrocarbon diesel fuel produced by refining petroleum, other liquid fuels obtained by the conversion of methane or coal have been under consideration since the 1920's. Methanol has been proposed as one such alternative fuel for internal combustion engines. Methanol is usually manufactured from carbon monoxide and hydrogen, which have historically been obtained in large volume from either natural gas or coal. Carbon monoxide can also be obtained from almost any carbon-containing substance, including agricultural and forest products and many waste materials. The large supply and wide distribution of raw materials for manufacturing methanol is responsible to a large degree for its growing use as a fuel for internal combustion engines. However, methanol has a very low heating or BTU value. Thus, the performance of an internal combustion engine declines considerably when methanol is employed as the fuel. By contrast, relative to methanol, dimethyl ether has a higher BTU value and is non-toxic. In addition, dimethyl ether is a clean-burning fuel whose combustion gases are essentially free of solid particles.
German Patent Number 654,470 (Dec. 20, 1937) to Otto Gross Wanne-Eickel describes mixtures of methanol and dimethyl ether containing from 5 percent to 45 percent methanol (and hence from 95 percent to 55 percent dimethyl ether) as being a suitable fuel to meet increasing fuel needs for internal combustion engines. Existing amounts of fuel available from petroleum were not sufficient at that time to meet such increasing fuel needs. Selection of dimethyl ether with a limited methanol content, instead of pure methanol, is said to be desirable because the specified mixtures can be used, generally, to fuel internal combustion motors without causing the considerable loss in performance of such motors using pure methanol for fuel. The patent also states, however, dimethyl ether itself has such a strong tendency to cause knocking in spark ignition motors that is not possible to achieve normal operation. No working examples or other supporting data are provided in this 1937 German patent.
Numerous methods have been disclosed for the production of dimethyl ether in combination with methanol and water from synthesis gas obtained from various sources, such as natural gas, coal or essentially any carbon-containing substance. Bell et al., U.S. Pat. No. 4,341,069; Van Dijk et al., U.S. Pat. No. 5,177,114; and published European Patent Applications Numbers 0324475 and 0409086 A1 are examples of such disclosures. In particular, European Patent Applications Numbers 0324475 and 0409086 A1 disclose how process conditions can be controlled in one such method in order to produce various mixtures of dimethyl ether and methanol having a wide range of mole ratios of dimethyl ether to methanol.
In numerous methods for the manufacture of dimethyl ether, dimethyl ether is produced in a product mixture that also contains methanol and/or water. Furthermore, removal of methanol and water from dimethyl ether in such a product mixture requires additional processing steps. Thus, it would be highly desirable to be able to employ mixtures of dimethyl ether, methanol and water--or, in other words, crude or unpurified dimethyl ether--directly as diesel fuels in order to avoid the aforesaid additional processing steps associated with purifying crude dimethyl ether and, ideally, so that process conditions could be employed in order to produce such mixtures directly from synthesis gas. In that way it would be possible to avoid or at least minimize the need for additional processing steps, such as purification steps, and still produce a highly effective and economical alternative diesel fuel.
U.S. Pat. No. 4,422,412 to John H. R. Norton describes modification of a compression ignition engine (diesel) to pass a portion of a methanol fuel stream directly into a cylinder of the engine and divert a portion of the methanol fuel stream to catalytic conversion of the diverted methanol to dimethyl ether and water in a reactor whose outlet is in communication with the same cylinder. The combined gaseous mixture in the cylinder is up to about 50 percent by weight, preferably from about 5 to 30 percent by weight dimethyl ether. Stoichiometry of this methanol conversion requires molar amounts of dimethyl ether and water formed to be equal. Therefore, a balance of the combined gaseous mixture in the cylinder is at least about 31.6 percent methanol (50 percent dimethyl ether, 18.4 percent water), and preferably about 58.9 to 93.2 percent methanol.
U.S. Pat. No. 4,603,662 to John H. R. Norton and Peter R. Rebello describes a fuel composition that contains a mixture of at least one ether and at least one alcohol, and optionally may contain water, normal diesel fuel, and cetane improvers. For use in a compression ignition engine the patent states that, generally, from 5 to 80 percent by volume, more usually from 5 to 20 percent by volume of fuel may be ethers. The fuel may contain small amounts of lubricants, e.g. up to about 2 percent by volume (more generally about 1 percent by volume) of an oil such as castor oil. Dimethyl ether and methanol is described as a convenient fuel because dimethyl ether is soluble in methanol (without water) at room temperature and pressures. U.S. Pat. No. 4,603,662 specifically illustrates fuels containing: (a) 5 percent of dimethyl ether by volume and 95 percent of methanol with and without an additional 1 percent of castor oil in Examples 1 and 3; and (b) 20 percent of dimethyl ether and 80 percent of methanol or 78 percent of dimethyl ether and 2 percent castor oil and as in Example 5.1.
Levine, U.S. Pat. No. 4,892,561, describes a first diesel fuel composition free of methanol that contains 95-99.9 percent by weight of dimethyl ether and 0.1-5 percent by weight of a cetane number-improving additive such as water. Addition of water to dimethyl ether, forming a mixture rather than a compound, is said to avoid formation of toxic methanol. This patent also discloses a second diesel fuel composition that contains at least 50 percent by weight of the aforesaid first diesel fuel and the remainder conventional hydrocarbon diesel fuel.
However, thus far, there has not been a disclosure of the compositions of mixtures of dimethyl ether, methanol and water that contain the balance of concentration levels of dimethyl ether, methanol and water necessary for the resulting diesel fuel to afford both environmental benefits and good ignition characteristics, that can be produced economically without the need for costly purification steps, and that can be maintained as a stable single liquid phase both in use and during storage.
It is therefore a general object of the present invention to provide an improved alternative diesel fuel composition which overcomes the aforesaid problems and affords the aforesaid benefits.
More specifically, it is an object of the present invention to provide an improved alternative diesel fuel composition that has a high BTU value.
It is another object of the present invention to provide an improved alternative diesel fuel composition that is a clean burning material whose overall emissions are lower and whose combustion gases are essentially free of solid particles.
It is a further object of the present invention to provide an improved alternative diesel fuel composition that affords good ignition characteristics.
It is another object of the present invention to provide an improved alternative diesel fuel composition that can be produced economically without the need for costly purification steps.
It is an additional object of the present invention to provide an improved alternative diesel fuel composition that is maintained in a stable single liquid phase both in use and during storage.
Other objects and advantages of the present invention will become apparent upon reading the following detailed description and appended claims.